[PECOFF] Move more code from Atoms.h to ReaderCOFF.cpp.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@187688 91177308-0d34-0410-b5e6-96231b3b80d8
Remove unused using's.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@187690 91177308-0d34-0410-b5e6-96231b3b80d8